In Vitro Antiprotozoal Activity of Hibiscus sabdariffa Extract against a Ciliate Causing High Mortalities in Turbot Aquaculture

Simple Summary The in vitro antiprotozoal activity of Hibiscus sabdariffa ethanolic extract was evaluated on the ciliate parasite Philasterides dicentrarchi, which causes severe losses in turbot aquaculture. Our results showed that H. sabdariffa extract had successfully inhibited the parasite population growth rate (IC50 = 1.57 mg mL(-1)), and caused significant changes in the activity of antioxidant enzymes (LOEC = 0.22 mg mL(-1)), especially glutathione peroxidase, total glutathione, and catalase. The activity of proteases (virulence factors) was also inhibited (IC50 = 0.76 mg mL(-1)), and the gene expression of catepsin 90 and leishmanolysin proteases was downregulated. Organic acids and phenolic phytochemicals in hibiscus extract are potentially responsible for the antiprotozoal bioactivity herein determined. Therefore, H. sabdariffa extract can be a promising disease-control alternative against the ciliate proliferation, cellular defense mechanisms, and pathogenicity. Philasterides dicentrarchi is an histophagous parasite that infects flatfish, namely turbot (Scophthalmus maximus), and cause significant losses in aquaculture units. The available measures for P. dicentrarchi control have limited efficiency, and some cause harm to fish. Hence, sustainable and natural control strategies are urgently needed. This study evaluated the in vitro bioactivity of the ethanol extract of Hibiscus sabdariffa calyces on P. dicentrarchi population growth rate (PGR), oxidative stress biomarkers (glutathione-S-transferases (GST), glutathione reductase (GR), glutathione peroxidase (GPx), total glutathione (TG) and catalase (CAT), neurotoxicity (acetylcholinesterase, AChE), activity and gene expression of proteases as major virulence factors. H. sabdariffa extract inhibited parasite PGR (IC50 = 1.57 mg mL(-1)), and caused significant changes in the activity of antioxidant enzymes (LOEC = 0.22 mg mL(-1)), especially GPx, TG, and CAT. The activity of proteases was also severely inhibited (IC50 = 0.76 mg mL(-1)), and gene expression of catepsin 90 and leishmanolysin proteases was downregulated. Organic acids and phenolic phytochemicals in hibiscus extract are potentially responsible for the antiprotozoal bioactivity herein determined. Therefore, H. sabdariffa extract can be a promising disease-control alternative against the ciliate proliferation, cellular defense mechanisms and pathogenicity. Still, its applicability in aquaculture settings, and potential effects on farmed fish, should be further elucidated.